In many display applications the visibility of details is of great importance, either since the fine details are of importance, such as in medical image processing, or simply to improve the visual appeal of the image. Sharpening of the image and local contrast enhancement methods are used to enhance the local contrast. Such a method is for instance known from “Locally adaptive image contrast enhancement without noise and ringing artifacts” by Sacha D Cvetkovic et al, Proc. of ICIP 2007, vol. p. 557-560, September 2007, San Antonio, Tex. In this method an algorithm is used that tries to equalize amplitudes of all output frequencies by assigning higher gains to frequencies with lower energy (amplitude) and vice versa. The algorithm is designed for to enhance images or video for surveillance applications to assist in the inspection of the images.
Local contrast enhancement methods suffer from annoying side effects such as halo and ringing. The known method aims to reduce these effects. The known method is however complicated and may lead to unpleasing effects providing an unnatural outlook.
It is an object of the invention to provide a method, conversion unit and image processing apparatus with an aim of increasing the quality of reproduction by relatively simple means and providing a more pleasant and natural appearance of images.